The Russians will have something to say about this. Now, Ukraine is, de facto, part of NATO, but fighting as a proxy without NATO sending their forces, en masse, to fight alongside Ukraine. The Russians, clearly, are taking nothing for granted and that's why they are building such a big and powerful army. They are readying themselves to take on NATO too, if they are forced to. But that would bring us to the very cusp of WW3 and the use of nuclear weapons. This is unlikely, as the United States would rather fight a war against China, where the stakes are far higher than Ukraine.